Masahiro, the first of his name at Saga, was the son of Yoshinobu, a pupil and son-in-law of the first-generation , which makes him the founder's grandson and the elder brother of Dewa no Kami Yukihiro. He first signed Masanaga and changed his name to Masahiro in the eleventh month of Kan'ei 2 (1625). After the founder's death he served the second-generation Omi no Daijo Tadahiro as a close attendant and assistant, and the published sources call him a most able collaborator of the main house. They also judge him the most skilled of all the collateral smiths, the waki-, 「傍肥前の中でも最も技量が優れており」. He is said to have died in 5 (1665) at the age of fifty-nine.
The date of his title is a matter the published record treats with care. Tradition places his appointment as Kawachi Daijo in Kan'ei 5, yet works dated as late as Kan'ei 16 are still signed no Saga-ju Masahiro or simply no Masahiro, and the title first appears on a dated the eighth month of Kan'ei 18. The modern view therefore places the appointment about Kan'ei 18, shortly after Tadahiro himself received the Omi no Daijo title, which accords with the tradition that Masahiro worked throughout as the assistant of the main house. One whose signature combines the features of his pre-title and post-title hands is valued for exactly this reason, as a work made soon after the appointment.
His manner stands against that of the main line. Where the house excels in , Masahiro preferred the irregular temper, and the published sources state it plainly: 「作風は乱れた刃を好んで焼き」. His typical is a mixed with and , and the clove pattern is likened to that of no Daijo Tadahiro. Several blades begin with a straight at the base, and in places the clusters of are linked by a low, shallow, -toned stretch, both of which the judges call a main point of the waki- temper. The temper is set high and low, long and enter well, the is deep and bright, and thick gathers in the valleys of the . and run through the , and on several blades the are recorded as notably long. with rounded heads are named as a point of his , and a wide temper of showy is called his own domain.
The beneath it is, on most blades, the refined forging: a tightly packed with , on some laid dust-fine and thick with fine entering, while a few pieces run instead an that stands. The runs straight to a and turns back with , at times slightly . The is often broad and long, and one with a greatly extended is described as powerful and full of movement. The published sources find in his work more natural vigor than in that of the main house, 「本家忠吉家の作よりも自然、覇気のある作が多く」.
He also commanded the , though it is comparatively rare in his record. One with a deep- , the evenly attached and the bright and clear, is singled out as among the finest of his works, the published sources concluding that it shows the height of his skill in as well as in . A Kan'ei 11 of mixed with small is named the finest of his works, 「この刀はその白眉である」. The judges add two points that separate him from the main line. His are signed on the , and his are , unlike those of the main house, becoming a large on early works whose tangs narrow toward the tip. His signature grows smaller in his last years.
Masahiro is graded Jo-jo by Fujishiro. He has no National Treasures and no Important Cultural Properties, and his record runs through the rank, where thirty of his works have been designated. Dated pieces survive in small numbers, from Kan'ei 9 through Keian 1 and Joo 4 to a of 4, his last years. Several carry carving by his compatriot Yoshinaga, and on one the Shigetsu Hotei in relief within the is called an extremely rare design among blades, paralleled only on a of no Daijo Tadahiro. Another is mounted in a Yagyu and carries the ownership inscription of Yagyu Renyasai Toshikane. Recorded provenance is otherwise slight. A signed Masahiro of the rank comes to light from time to time, with patience, as the work of the most accomplished of the waki-.